In the automobile, shipbuilding and construction industries, “multi-material structures” which comprise plastics on the one hand and metals on the other hand are being used more and more frequently, the two material constituents being frictionally connected to one another. In this way, the properties of both materials are combined in order thus to obtain a composite material having improved properties. In order to connect the plastics and the metals to one another, first of all mechanical connections and adhesive connections may be used.
Suitable mechanical connections are in particular riveted or clinch connections, which, however, have the following disadvantages. Firstly, high stress concentrations occur in the workpieces. Secondly, subsequent slackening of the connection may occur as a result of creep, moisture and relaxation. Finally, a disadvantage with mechanical connections is the fact that their external appearance rules out use in the visible region or it is necessary for these regions to be subsequently processed.
Adhesive connections have the disadvantage that the strength of the connection can be estimated only with difficulty. Furthermore, the problem arises that the cycle times for producing an adhesive connection are comparatively long due to the curing time of an adhesive. In addition, solvents may be released during the curing, meaning that monitoring of emissions may become necessary when producing the adhesive connection. In addition, if increased temperatures are required for the curing, the workpieces may also be affected. Finally, for a reliable adhesive connection, it may be necessary to pretreat the surfaces to be connected, which means an additional outlay.
In addition to the mechanical connections and the adhesive connections, welding processes are also known in order to produce multi-material structures. The welding processes applied for this purpose comprise ultrasonic welding, resistance welding, vibration welding, induction welding and infrared welding. In this case, however, the following main disadvantages become evident. Firstly surface pretreatment is necessary here, too, and secondly the energy consumption for producing such a welded connection is relatively high on account of the comparatively low efficiency. In addition, the connections produced in this way have only low reliability, meaning that they cannot be used in sensitive regions.
In the meantime, “hybrid joining techniques” have been developed in which it is attempted to improve the connection properties by the combination of two or more conventional joining methods. To this end, DE 101 49 633 A1 discloses metal-collar joining, a hole first of all being punched in the metal part, so that the hole is subsequently surrounded by a circular metal “collar”. This collar is then pressed into the plastic part to be connected to the metal part, so that there is then a positive-locking connection. During this pressing-in, however, hairline cracks may occur in the plastic part, and these hairline cracks in turn may lead to the failure of the connection under load.
In another hybrid method, a metal and a plastic are connected to one another in such a way that the plastic is injected into cavities in the course of an injection moulding process, the cavities having been formed in the metal parts beforehand, so that a positive-locking connection is produced between plastic on the one hand and metal on the other hand. Due to the complicated preliminary processing of the metal parts, however, such a method is very time-consuming and requires long cycle times.